It was not possible to say what the source of the DNA. They use the term body fluid. In conclusive but 15 out of the 19 alleles for Madeleine were mixed in the mixed DNA sample that had at least 3 contributors. That is a strange result.
I have used the word blood because that's what the CSI dog was trained to find.
I have often wondered why the sample contained DNA from more than one person. I can understand that happening in a case of gang-rape, for example, but not quite how it happened in this case. If the CSI dog was as good as Grime said, there was at least some blood in the boot. Then what? Two other people bled or dribbled on exactly the same spot?
The other problem is that DNA analysis is seen as an exact science but it isn't. Different analysts can reach different conclusions about the same sample, particularly when it is from more than one person.
A man was convicted in a gang-rape case in the US because the experts said his DNA couldn't be excluded from a mixed sample. The same sample was later given to 17 analysts for retesting. Only one agreed with the original finding; that he couldn't be excluded. 4 said the evidence was inconclusive and 12 said he could be excluded.
https://www.newscientist.com/article/mg20727733.500-fallible-dna-evidence-can-mean-prison-or-freedom/The 'inconclusive' conclusion reached by the FSS may have been correct, but a different conclusion could have been reached by a different analyst in my opinion.